Tysabri Effects on Cognition and Neurodegeneration in Multiple Sclerosis

Inclusion Criteria:

18 through 60 years of age inclusive
Diagnosis of relapsing remitting multiple sclerosis
Prior to treatment phase, have had disease activity with at least 1 documented relapse during the previous year OR 2 documented relapses during the previous 2 years OR one or more new MRI lesions (Gd+ and/or T2 hyperintense)
An Expanded Disability Status Scale (EDSS) score of 0-4.5 inclusive
Neurologically stable with no evidence of relapse or corticosteroid treatment within 30 days prior to treatment
Never been treated with Tysabri/natalizumab.

Exclusion Criteria:

Another type of MS other than relapsing remitting multiple sclerosis (RRMS)
A history of chronic disease of the immune system other than MS or a known immunodeficiency syndrome/immunocompromised
A history or presence of cancer (except for successfully treated basal or squamous cell carcinoma of skin)
Active systemic bacterial, viral or fungal infections, or diagnosis of AIDS, Hepatitis B, Hepatitis C infection defined as a positive HIV antibody, Hepatitis B surface antigen or Hepatitis C antibody tests, respectively
Have received any live or live attenuated vaccines (including for varicella-zoster virus or Measles) within the last 2 months
Have received total lymphoid irradiation or bone marrow transplantation
Have been treated with: corticosteroids or adrenocorticotropic hormones (ACTH) within the last month, IFN-β or glatiramer acetate within the last 3 months, immunosuppressive medications such as azathioprine or methotrexate within the last 6 months, immunoglobulins and/or monoclonal antibodies (including natalizumab) within the last 6 months, or cladribine, cyclophosphamide or mitoxantrone at any time.
Any medically unstable condition or a progressive neurological disorder, other than MS, which may affect participation in the study
History of any clinically significant cardiac, endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic, dermatologic, psychiatric, renal or other major disease
Unable to undergo MRI scans, including claustrophobia, have a pacemaker or history of hypersensitivity to gadolinium-DTPA
Have had a relapse within 30 days prior AND/OR not stabilized from a previous relapse
History of severe allergic or anaphylactic reactions or known drug hypersensitivity to natalizumab/Tysabri
A clinically significant infectious disease, such as cellulitis, pneumonia, septicemia
History of progressive multifocal leukoencephalopathy(PML)
Participation in any clinical research study evaluating another investigational drug or therapy within the last 6 months
History of Tysabri therapy
Abnormal screening blood test
Females who are not postmenopausal for at least 1 year, surgically sterile or willing to practice effective contraception during the study
Nursing mothers, pregnant women, and women planning to become pregnant while on study